1. Field of the Invention
The present invention relates to an N-type organic thin film transistor, an ambipolar field-effect transistor, and methods of fabricating the same and, more particularly, to an N-type organic thin film transistor and an ambipolar field-effect transistor using silk protein as the gate-insulating layer, and methods of fabricating the same.
2. Description of Related Art
As well known to those skilled in the art, transistors are applied in a wide variety of electronics to serve as switches for electric current, e.g. valve for controlling electric current. Different from mechanical valves, transistors are controlled by electric signals and the switch-speed of the transistors can be very fast. Transistors, for examples, may be classified into bipolar junction transistors (BJTs) and field effect transistors (FETs). The field effect transistor comprises N-type organic thin film transistor (OTFT) and P-type organic thin film transistor, etc.
Usually, the electron mobility of an N-type organic thin film transistor is smaller than that of a P-type organic thin film transistor. For example, most of the N-type organic thin film transistors have an electron mobility of less than 1 cm2/Vs. For a CMOS device, both N-type organic thin film transistors and P-type organic thin film transistors are used and are required to have high mobility. Since the N-type organic thin film transistors have low mobility, there is a present need to put efforts towards developing a novel N-type organic thin film transistor.
Organic Thin film transistors (whether N-type or P-type) can be classified into top contact organic thin film transistor (top contact OTFT) and bottom contact organic thin film transistor. As shown in FIG. 1A, a top contact organic thin film transistor comprises: a substrate 10; a gate electrode 11 locating on the substrate 10; a gate-insulating layer 12 disposed on the substrate 11 and covering the gate electrode 11; an organic semiconductor layer 13 covering the entire surface of the organic semiconductor layer 12; and a source electrode 14 and a drain electrode 15 disposed on the organic semiconductor layer 13.
In addition, as shown in FIG. 1B, the bottom contact OTFT comprises: a substrate 10; a gate electrode 11 disposed on the substrate 10; a gate insulating layer 12 disposed on the substrate 10 and covering the gate electrode 11; a source electrode 14 and a drain electrode 15 disposed on the gate insulating layer 12; and an organic semiconductor layer 13 entirely covering the gate insulating layer 12, the source electrode 14, and the drain electrode 15.
According to academic assays, fullerene (C60) has high electron mobility so can be used as a semiconductive material for N-type OTFTs (N-type organic thin film transistor). In 2006, Kenji Itaka et al. used aluminum oxide as the gate-insulating layer and used pentacene in the buffering layer to improve the efficiency of the OTFTs (Kenji Itaka,* Mitsugu Yamashiro, Jun Yamaguchi, Masamitsu Haemori, Seiichiro Yaginuma, Yuji Matsumoto, Michio Kondo, and Hideomi Koinuma, “High-Mobility C60 Field-Effect Transistors Fabricated on Molecular-Wetting Controlled Substrates” Adv. Mater. 2006, 18, 1713-1716), in which the electron mobility can be about 4.91 cm2/Vs. However, the fullerene thin film is not compact enough and therefore water or oxygen may be easily diffused into fullerene thin film, which means the resultant OTFT (organic thin film transistor) is unstable in the air, has poor reliability, and has low commercial value.
In the same year (2006), Thomas D. Anthopoulosa et al. used Divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as an insulating-layer and used fullerene (C60) during sputtering to increase electric properties of OTFTs (Thomas D. Anthopoulosa_Birendra Singh, Nenad Marjanovic, Niyazi S. Sariciftci, Alberto Montaigne Ramil, Helmut Sitter, Michael Cone and Dago M. de Leeuw, “High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films”, APPLIED PHYSICS LETTERS 89, 213504 (2006)). The as provided OTFT has an electron mobility of about 6 cm2/Vs. However, the fullerene (C60) thin film is not compact enough and is not stable in the air, and therefore the reliability of the OTFT cannot be improved and the commercial value cannot be increased.
Hagen Klauk proposed a report about N-type OTFT (at the left column on page 2605 of “Organic thin-film transistors”, Chem. Soc. Rev., 39, 2643-2666 (2010)), which mentioned that there is presently no new material and/or new structure which can improve the electron mobility and stability of C60 N-type OTFT in the air.
Therefore, it is desirable to provide an improved material and/or new structure which can make N-type OTFT in an easy way and enable the electron mobility enhancement of the N-type OTFT. A CMOS with both P-type and N-type OTFTs may exhibit excellent efficiency as a consequence.
Moreover, ambipolar field-effect transistor is a kind of transistor, which possesses both electron and hole transportations. An ambipolar field-effect transistor has a structure similar to an OTFT but the device characteristics are slightly different. The development of novel material and/or structure in the configuration of ambipolar field-effect transistors is also of great commercial value if the stability and mobility can be greatly improved.